1. Field of the Invention
This invention relates to a radiation-shielding transparent material and a method of producing the same, which material shields radioactive rays such as neutron beams and .gamma.-rays leaking from nuclear reactors, cyclotrons, or the like.
2. Description of the Prior Art
Neutron beams and .gamma.-rays leaking from nuclear reactors, cyclotrons, or the like collide with surrounding substances and cause radiation which may be hazardous to people and apparatuses exposed thereto.
The leaking neutrons are in any form of high-speed neutrons, low-speed neutrons, and thermal neutrons. To moderate the high-speed neutrons, use of elements with small atomic numbers and compounds thereof has been known, such as hydrogen, helium, lithium, beryllium, boron, carbon, nitrogen, water, heavy water, and the like. Effective moderation of the high-speed neutrons is achieved by their collision with hydrogen (with an atomic number 1) having a small mass which is similar to that of neutrons, so that materials having a high concentration of hydrogen are very effective in moderating the high-speed neutrons. Water whose molecule has two hydrogen atoms and one oxygen atom is a least expensive yet very effective moderator for shielding the high-speed neutrons. More particularly, the high-speed neutrons are moderated by collision with water and converted into low-speed neutrons and thermal neutrons. However, the elements with a small atomic number and a small mass and compounds thereof are not effective in shielding .gamma.-rays, and only elements with a large atomic number and compounds thereof are effective in shielding .gamma.-rays, such as tungsten, lead, thallium, bismuth, tantalum, thorium, plutonium, and the like.
The low-speed neutrons and thermal neutrons are moderated by elements having a large cross section for neutron absorption, such as boron, cadmium, indium, and the like, so that the low-speed neutrons and thermal neutrons are converted by the moderation into .gamma.-rays having an energy of about 0.42 MeV, whereby the overall energy of the leaking radiation is attenuated.
Conventionally, heavy concrete containing a moderator, such as iron, lead, barium, metal hydride, serpentine, boron, and the like has been used to shield nueclear reactors, cyclotrons, and the like. The heavy concrete is highly effective in absorbing .gamma.-rays but not so effective in moderating neutrons which are leaking from the nuclear reactors, cyclotrons, or the like.
However, no materials capable of effectively moderating both .gamma.-rays and neutrons without being damaged thereby have been found yet.
Lead glass has been used as a material for checking windows of nuclear reactors, cyclotrons, and the like, and the lead glass has been known as an effective radiation-shielding transparent material. Nevertheless, the transparent lead glass as the radition-shielding material has shortcomings in that the lead glass is very costly, i.e., one hundred million yen per several cubic meters thereof; that the lead glass is brittle when being machined, so that it has been difficult to machine a lead glass member into desired dimensions with high accuracy; and that the lead glass is coloured with the increase of lead content therein and the transparency thereof is reduced by the colouring. Thus, there have been no radiation-shielding transparent materials, except the lead glass, which are suitable for total absorption calorimeters for measuring the total energy of .gamma.-rays and for shielding nuclear reactors and cyclotrons. The lack of radiation-shielding transparent materials overcoming the aforesaid shortcomings of the lead glass has seriously hampered the research and development of the aforesaid apparatuses of nuclear industries.
As regards radiation-shielding materials which are less costly than the lead glass, a solution of zinc bromide (ZnBr.sub.2) has been known, but such a solution has shortcomings in that long-term chemical stability thereof is low and that the transparency thereof is gradually deteriorated. Accordingly, such solutions are seldom used now due to the aforesaid shortcomings.
Thus, there has been a pressing need for development of a radiation-shielding transparent material overcoming the shortcomings of the lead glass, so as to further expand the practical applications of radiation-related apparatus: for instance, shielding of nuclear reactors, instruments for measuring radiations such as .gamma.-rays and neutrons, and medical apparatuses using x-rays and .gamma.-rays.